This invention relates to a process for the electrolytic anodic coupling of alkyl polyalkylbiphenyldicarboxylates and the polyalkylbiphenyldicarboxylic acids prepared thereby. Ortho polyalkylbiphenyldicarboxylic acids are desirable starting materials for polyester resins, including alkyds, and for plasticizers. Meta polyalkylbiphenyldicarboxylic acids are desirable starting materials for polyester resins including alkyds, polyamide resins and fiber-forming polymers with outstanding properties for certain applications. It is known that the substituted biphenyl grouping reduces flexibility and increases rigidity.
Alkylbiphenylpolycarboxylic acids can be obtained by standard procedures using the classical Ullmann coupling reaction followed by complete oxidation of all the alkyl groups but the selective oxidation of the ortho and meta groups has not been successful. Moreover, the Ullmann reaction requires a halogen in the position on the aromatic ring where the coupling is to take place. Electronegative groups, such as nitro and methoxycarbonyl, particularly in the ortho position, strongly activate an aryl halogen. But on the other hand, the reaction is greatly inhibited or prevented by the presence of substituents which provide an alternative path for reaction of the aryl halide, such as amino, hydroxy, and free carboxyl groups. Bulky groups in the ortho positions may impede the reaction by steric hindrance. At best, the Ullmann reaction is a two-step reaction in that it requires the positioning of a halogen at the point on the ring where coupling is to take place, and coupling can be impeded because of ring substituents.
The oxidative coupling of monosubstituted benzenes with palladium salts has been reported by R. Van Helden and G. Verbug (Recueil, 84, 1965, 1263-1273). However, with di- and trisubstituted benzenes, steric factors were found to play an important role. From ortho xylene, 3,4,3',4' and 2,3,3',4' tetramethylbiphenyl were obtained as the only products. Mesitylene could not be converted into the corresponding biphenyl as might be expected. Thus, oxidative coupling with palladium catalyst has been reported to be handicapped by steric factors or the product tends to be of the para isomers rather than the ortho or meta isomer.
The electrolytic coupling of polymethyl benzene (mesitylene) to its biphenyl derivatives has been reported by L. Eberson and K. Nyberg ("Anodic Oxidations", ACS Div. of Pet. Chem., Chicago, Vol. 15, No. 4, Sept. 1970, B7). These investigators found that biaryl coupling was generally the major reaction mode for anodic oxidation of 1,3,5-trimethyl benzene (mesitylene) in non-nucleophilic (non-electron donating) media with a suitable electrolyte. Another type of anodic oxidation process is the well-known Kolbe synthesis whereby the anodic oxidation of a carboxylate structure is followed by decarboxylation and coupling to yield a di-grouping of the hydrocarbon or alkyl moiety in the carboxylate structure. It is equally well known that aromatic carboxylic acids in which the carboxyl group is directly attached to a benzene ring fail to undergo the Kolbe reaction to any extent, as is reported by B. C. L. Weedon ("Anodic Syntheses With Carboxylic Acids" Quarterly Reviews Vol. 6, No. 4, 1952 p. 387-388). Benzoic acid on electrolysis in methanol yields benzene rather than undergoing a coupling reaction. The electrolysis of 4-t-butyl-2,6-dimethyl benzoic acid in methanol gives 3-methyl-4-t-butyldimethyl phthalate and 1,3-dimethyl-5-t-butylbenzene rather than undergoing coupling.
We have now found that methyl polymethyl benzoates can be made to couple with the ester groups in the ortho and meta positions. This process is capable of producing the desired ortho and meta methyl dicarboxylates of polymethyl substituted biphenyls in good yields. These biphenyls have been prepared and there is evidence of one terphenyl in a gas chromatograph peak.